Hyponatremia

Excerpt

A crucial development that enabled animals to migrate from an aquatic to a terrestrial environment is the regulation and conservation of water and salt during periods of excess and scarcity. Hyponatremia is the most common electrolyte abnormality found in hospitalized individuals. This condition is usually defined as a serum sodium concentration of less than 135 mEq/L, but may vary slightly depending on the laboratory's set values. An excess of total body water usually causes hyponatremia relative to total body sodium content.

Sodium and water balance depend on a complex interplay between volume status, antidiuretic hormone, kidney function, cardiac function, natriuretic peptides, and other hormones. Hyponatremia represents an imbalance in this ratio, in which total body water exceeds total body solutes. Total body water comprises 2 main compartments: extracellular fluid (ECF), which accounts for one-third, and intracellular fluid (ICF), which accounts for the remaining two-thirds. Sodium is the primary solute of ECF, and potassium is the primary solute of ICF.

Hyponatremia can be classified as follows:

1. Mild is 130–135 mEq/L.

2. Moderate is 125–130 mEq/L.

3. Severe is <125 mEq/L.

Patients with hyponatremia are classified into 3 main categories: hypovolemic, euvolemic, and hypervolemic. In addition, patients can be categorized by tonicity as hypotonic, eutonic, or hypertonic. Tonicity is defined as the amount of effective osmoles that cannot cross the cellular membrane from the extracellular to the intracellular space and therefore influence the movement of water across cell membranes. This differs from serum osmolality, which includes all solutes and is defined as 2 x [Na] + [glucose]/18 + [blood urea nitrogen]/2.8.

Osmolality is dependent on the properties of the solution and independent of movement across a membrane. Tonicity excludes urea, which freely crosses the cellular membrane and can also be considered a measurement of effective osmoles. Tonicity is defined as 2 x [Na] + [glucose]/18. In normoglycemia, glucose can be excluded as its contribution to tonicity is minimal (5–10 mOsm/kg). Normal tonicity is 285 to 295 mOsm/kg; hypertonicity is more than 295 mOsm; hypotonicity is less than 285 mOsm/kg.

Most hyponatremic individuals will have hypotonic hyponatremia. Exceptions include those with hyperglycemia, those who received mannitol or immunoglobulins, and those with pseudohyponatremia. Understanding and treating hyponatremia is important as even mild hyponatremia has been shown to cause increased morbidity and mortality, particularly in older adults.